Although peer friendships provide the basis of support for most adolescents, characteristics of these friendships may also confer risk to depression, and may help to explain the emergence of gender differences in depression during adolescence. Theorists have hypothesized that the tendency to co-ruminate with one's friends may reflect a mechanism by which peers inadvertently foster cognitive vulnerability for depression amongst themselves, while simultaneously bolstering the friendship. Therefore, it may be a socially reinforcing tendency that increases risk for depression in adolescents. Previous research has supported the hypothesis that co-rumination increases risk for depression;however, the mechanisms by which this risk is conveyed remain unclear. The proposed study seeks to integrate cognitive and interpersonal models of risk in a prospective study of adolescent freshmen during their first semester transition into high school. Primary Aim 1 is to extend prior work on co- rumination by examining both adaptive and maladaptive outcomes in adolescents. We hypothesize that friends who co-ruminate will report higher-quality friendships while also reporting more depressive symptoms over time. In addition, we predict that the impact of co-ruminating with one's friends for depression risk is mediated by increased levels of rumination in adolescents. Further, we predict that this integrated model, which incorporates both interpersonal and cognitive components, will help to account for girls'heightened risk for depression beginning in adolescence. Primary Aim 2 is to evaluate these effects within the context of adolescents'social networks. Theorists have hypothesized that the similarity of behaviors observed between peers (including depressive symptoms) may be attributed to both selecting peers similar to oneself (selection effects) and adopting or developing peers'behaviors (contagion effects). To date, research on peer contagion for depression has been largely limited to the dyadic peer context. The proposed study will apply social network modeling to map depression contagion effects in the larger context of adolescents'peer networks. A key innovation of social network analyses is the ability to locate where dysphoric adolescents lie within their peer networks and to identify characteristics of individuals who are more versus less vulnerable to peer contagion effects from dysphoric peers. In the proposed study, we will evaluate the role of co-rumination as a mechanism for peer contagion of depression within adolescents'social networks. Further, we will determine whether the impact of co-rumination on peer contagion effects is specific to depression versus externalizing behaviors. The results of the proposed study will help to determine specific mechanisms of risk in at-risk adolescents, which is a necessary step in designing more focused interventions for those at greatest risk for depression. PUBLIC HEALTH RELEVANCE: Characteristics of adolescents'friendships may increase their risk for depression, particularly among adolescent girls. The proposed study represents an important step in understanding the peer context of depression risk, and will provide information on a specific mechanism that may help to explain the emergence of gender difference in depression during adolescence. Results from this study, therefore, may inform the development of more targeted prevention/ early intervention programs for those at greatest risk with the goal of both preventing depression onset and fostering healthier friendships.